1. Field of the Invention
The present invention generally relates to Ni-based catalysts and processes that employ such catalysts for catalytically converting light hydrocarbons (e.g., natural gas) to synthesis gas. More particularly, the invention relates to stabilized Ni-containing catalysts that are active for catalyzing the selective partial oxidation of methane and other light hydrocarbons to CO and H2.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes. Present day industrial use of methane as a chemical feedstock typically proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widely used process, or by dry reforming. Steam reforming proceeds according to Equation 1.CH4+H2O⇄CO+3H2  (1)Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The partial oxidation of hydrocarbons, e.g., natural gas or methane is another process that has been employed to produce syngas. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to the steam reforming processes, which are endothermic. Partial oxidation of methane proceeds exothermically according to the following reaction stoichiometry:CH4+1/2O2→CO+2H2  (2)
In the catalytic partial oxidation processes, natural gas is mixed with air, oxygen or oxygen-enriched air, and is introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2. This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. Furthermore, oxidation reactions are typically much faster than reforming reactions. This makes possible the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by the existing catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
A number of process regimes have been described in the literature for the production of syngas via catalyzed partial oxidation reactions. The noble metals, which typically serve as the best catalysts for the partial oxidation of methane, are scarce and expensive. The more widely used, less expensive, catalysts have the disadvantage of promoting coke formation on the catalyst during the reaction, which results in loss of catalytic activity. Moreover, in order to obtain acceptable levels of conversion of gaseous hydrocarbon feedstock to CO and H2 it is typically necessary to operate the reactor at a relatively low flow rate, or space velocity, using a large quantity of catalyst.
For successful operation at commercial scale, however, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of economical catalysts allowing commercial performance without coke formation. Not only is the choice of the catalyst's chemical composition important, the physical structure of the catalyst and catalyst support structures must possess mechanical strength and porosity, in order to function under operating conditions of high pressure and high flow rate of the reactant and product gasses. Another continuing objective in this field is to develop stronger catalysts and catalyst supports that do not cause a high pressure drop when subjected to high pressure reactant gases.
Of the methods that employ nickel-containing catalysts for oxidative conversion of methane to syngas, typically the nickel is supported by alumina or some other type of ceramic support. For example, V. R. Choudhary et al. (J. Catal., Vol. 172, pages 281-293, 1997) disclose the partial oxidation of methane to syngas at contact times of 4.8 ms (at STP) over supported nickel catalysts at 700 and 800° C. The catalysts were prepared by depositing NiO—MgO on different commercial low surface area porous catalyst carriers consisting of refractory compounds such as SiO2, Al2O3, SiC, ZrO2 and HfO2. Catalysts were also prepared by depositing NiO on the catalyst carriers with different alkaline and rare earth oxides such as MgO, CaO, SrO, BaO, Sm2O3 and Yb2O3.
U.S. Pat. No. 5,500,149 (assigned to British Gas plc) describes a Ni/Al2O3 catalyst that catalyzes the reaction CO2+CH4→2CO+2H2, and demonstrates how reaction conditions can affect the product yield. The partial oxidation of methane to synthesis gas using various transition metal catalysts under a range of conditions has been described by Vernon, D. F. et al. (Catalysis Letters 6:181-186 (1990)). European Pat. App. Pub. No. 640561 discloses a catalyst for the catalytic partial oxidation of hydrocarbons comprising a Group VIII metal on a refractory oxide having at least two cations. Multimonolith combustors are discussed by M. F. M. Zwinkels, et al. in a chapter entitled “Catalytic Fuel Combustion in Honeycomb Monolith Reactors” (Ch. 6, A. Cybulski et al., eds., STRUCTURED CATALYSTS AND REACTORS. 1998. Marcel Dekker, Inc., pp.149-177.)
European Patent No. EP 303,438 (assigned to Davy McKee Corporation) describes a catalytic partial oxidation process for converting a hydrocarbon feedstock to synthesis gas using steam in addition to oxygen. Certain high surface area monoliths coated with metals or metal oxides, e.g., Pd, Pt, Rh, Ir, Os, Ru, Ni, Cr, Co, Ce, La, which are active as oxidation catalysts, are employed in that process. Other suggested coating metals are noble metals and metals of groups IA, IIA, III, IV, VB, VIB, or VIIB of the periodic table of the elements.
M. B. Davis et al. discloses that in the presence of excess oxygen, bulk Ni is relatively inert as a catalyst for oxidation of methane in air at temperatures of about 1,000° C., while Pt and Pt—Rh are catalytically active (Combustion and Flame (2000) 123: 159-174). Those investigators showed that 40-mesh Ni gauze did not ignite and there was no conversion of methane under methane partial oxidation conditions, however once temperatures greater than 1,375° K were reached, a homogeneous ignition was apparent.
U.S. Pat. Nos. 3,957,682 and 4,083,799 (assigned to Texaco, Inc.) disclose an Iconel metal screen consisting of about 50-95% nickel that is a methane steam reforming catalyst. In these processes the Ni catalyst is initially activated by heating in an oxygen-containing gas. Similarly, U.S. Pat. No. 5,112,527 (assigned to Amoco Corporation) also describes Ni as a reforming catalyst in the presence of steam, a gaseous lower alkane and air and in combination with a Group VIII metal having partial oxidation activity.
Japanese Pat. App. No. S59-184701 (assigned to Hiroshima Laboratory) describes certain Ni—Cr and Ni—Mb alloy powder catalysts that are active as methanol reformers.
Liao, M.-S., et al. (“Dissociation of methane on different transition metals,” J. Mol. Catal. A: Chem. (1998) 136:185-194) give a theoretical comparison between Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag and Au. Those authors concluded that transition metals are very active, and coinage metals are inactive for generating CO and H2 from methane. Methane dissociation in the presence of adsorbed oxygen was also modeled.
J. Nakamura et al. (Sekiyo Gakkaishi (1993) 36:97-104) discuss the production of syngas by partial oxidation of CH4 over Group VIII metal catalysts. A variety of SiO2-supported metals were studied. Rh, Ru and Ni produced CO2 and H2O below 700° K, whereas CO and H2 were produced above 700° K via CO2/H2O reforming of excess CH4. Pt at 900° K also produced syngas, but reverse water-gas shift was active on this metal. Fe and Co only produced CO2 and H2O.
T. Hayakawa et al. (Sekiyo Gakkaishi (1996) 39:314-321) describe certain (La,Sr)(Co,Ni) oxide catalysts, having a perovskite structure, that are said to be active for the oxidative conversion of methane to synthesis gas. The stable activity of La0.8Sr0.2Co0.8Ni0.2O3-6 catalyst is said to be likely due to high dispersion of Ni metal and the presence of La2O3 and SrO as the carriers of the Ni catalyst.
U.S. Pat. No. 5,149,464 is directed to a method for selectively oxygenating methane to carbon monoxide and hydrogen by bringing the reactant gas mixture at a temperature of about 650° C. to 900° C. into contact with a solid catalyst which is generally described as being either:                a) a catalyst of the formula MxM′yOz, where:        M is at least one element selected from Mg, B, Al, Ln, Ga, Si, Ti, Zr and Hf; Ln is at least one member of lanthanum and the lanthanide series of elements;        M′ is a d-block transition metal, and each of the ratios x/y and y/z and (x+y)/z is independently from 0.1 to 8; or        b) an oxide of a d-block transition metal; or        c) a d-block transition metal on a refractory support; or        d) a catalyst formed by heating a) or b) under the conditions of the reaction or under non-oxidizing conditions. The d-block transition metals are said to include those having atomic number 21 to 29, 40 to 47 and 72 to 79 (i.e., the metals Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt and Au).        
U.S. Pat. No. 5,997,835 (assigned to Haldor Topsoe A/S) discloses that addition of small amounts of gold to a nickel-containing catalyst provides a catalyst with suppressed carbon deposition during steam reforming of hydrocarbons. Although gold decreases the catalytic activity of the nickel catalyst, the catalyst still provides sufficient activity for steam reforming. The gold and nickel are said to form a type of surface alloy.
Soviet Union Patent No. 1189500 (N. N. Kundo et al.) describes certain Ni or Ni—Cr alloy catalysts for air-O2 conversion of CH4 at atmospheric pressure. The catalyst is prepared by treating a polyurethane foam matrix with a suspension of Ni powder in aqueous carboxymethylcellulose, and then calcining in a reducing atmosphere.
WO 99/35082 (assigned to Regents of the University of Minnesota) describes certain unsupported transition metal monolith catalysts for catalyzing the partial oxidation of methane to synthesis gas. Transition metals said to be useful are Fe, Os, Co, Rh, Ir, Ni, Cu, Pd, Pt and mixtures thereof. Nickel supported on alumina monoliths was not found to be useful for syngas production.
U.S. Pat. No. 5,648,582 (assigned to Regents of the University of Minnesota) describes certain ceramic monolith supported rhodium, nickel and platinum catalysts for the catalytic partial oxidation of methane in gas phase at very short residence time (800,000 to 12,000,000 h−1). Increasing the catalyst temperature improved selectivities for CO and H2 products and improved methane conversion.
Li, Yu and Shen (J Fuel Chem and Tech (2001) 29:112-115) describe the effects of increasing pressure on partial oxidation of methane to syngas using nickel on an alumina support as the catalyst. It was concluded that at high pressure, the partial oxidation of methane is thermodynamically unfavorable. Not only does it affect the selectivities to H2 and CO, high pressure also restrains the decomposition of CH4 over the NiO form of catalyst. Decomposition of CH4 over the Ni0 form of the catalyst occurs quickly, however.
In order to operate at very high flow rates, at high pressure and using smaller catalyst beds in the smaller, short contact time (i.e., millisecond range) catalytic partial oxidation (CPOX) reactors, the catalysts should be highly active, have excellent mechanical strength, resistance to rapid temperature fluctuations and thermal stability at partial oxidation reaction temperatures. Conventional Ni-based catalysts suffer from Ni metal loss and carbon formation, which prevents their use under high pressure and high flow rate conditions, which are required for short contact time CPOX processes. Presently, most Ni-based syngas catalysts are promoted with precious metals such as Rh or Pt, to deter coking. In addition to being very expensive, those catalysts usually operate at high reaction temperatures of more than 1,000° C. and tend to promote steam reforming. Accordingly, there is a continuing need for more commercially attractive catalyst compositions for the catalytic partial oxidation of hydrocarbons, particularly methane, or methane containing feeds, in which the catalyst retains a high level of activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity, elevated pressure and temperature during extended use on stream.